Two-core optical fibers for distributed fiber sensors and systems

Abstract

A two-core optical fiber is provided for use in Brillouin distributed fiber sensor applications and systems. The two-core fiber includes a first and second core. Each core is configured to exhibit a Brillouin frequency shift greater than 30 Mhz relative to the other core. Further, each core possesses temperature and strain coefficients that differ from the other core. The cores can be configured to produce Brillouin frequency shift levels of at least 30 Mhz relative to one another. These differences in shift levels may be effected by adjustment of the material compositions, doping concentrations and/or refractive index profiles of each of the cores. These optical fibers may also be used in BOTDR- and BOTDA-based sensor systems and arrangements.

What is claimed is: 
     
       1. An optical fiber comprising:
 a first core, a second core, and a cladding surrounding both of the cores, wherein at least one of the following conditions is satisfied: 
 (i) each core is configured with a refractive index profile that is substantially uniform along its length and differs from the refractive index profile of the other core; 
 (ii) each core is configured with a doping concentration that differs from the doping concentration of the other core; 
 (iii) each core has a different composition than the other core; 
 (iv) the optical fiber further comprises a refractive index ring configured to surround at least one of the cores; 
 (v) the cores are configured such that the distance between the center of the cores is at least 25 μm; and 
 (vi) the cores are configured such that the distance between the center of the cores is at least 25 μm and each core is surrounded by a low refractive index ring; 
 wherein each core is configured to exhibit a single Brillouin frequency shift peak (BFS peak) when light is launched into the core, and further wherein the BFS peak of each core differs from the BFS peak of the other core in terms of frequency and each core is configured to exhibit a Brillouin frequency shift of at least 30 Mhz relative to the other core. 
 
     
     
       2. The fiber according to  claim 1 , wherein each core is configured to exhibit a Brillouin frequency shift of at least 80 Mhz relative to the other core. 
     
     
       3. The fiber according to  claim 1 , wherein the optical fiber further comprises a refractive index ring configured to surround at least one of the cores. 
     
     
       4. The fiber according to  claim 1 , wherein the cores are configured such that the relative refractive index of the cores to the cladding is between about 0.2% to 2%. 
     
     
       5. The fiber according to  claim 1 , wherein the refractive index profile for each core is selected from the group consisting of step-shaped and graded index profiles. 
     
     
       6. The fiber according to  claim 1 , wherein each core is doped with germania. 
     
     
       7. The fiber according to  claim 6 , wherein each core is doped with approximately 1 to 10 mol % germania. 
     
     
       8. The fiber according to  claim 3 , wherein a trench separates the refractive index ring from the at least one of the cores. 
     
     
       9. The fiber according to  claim 1 , wherein the cores are further configured such that the distance between the center of the cores is at least 25 μm. 
     
     
       10. The fiber according to  claim 6 , wherein each core is doped with a different concentration of germania such that the absolute difference in dopant concentration between the cores is about 0.5 mol % or greater. 
     
     
       11. The fiber according to  claim 10 , wherein each core is doped with a different concentration of germania such that the absolute difference in dopant concentration between the cores is about 1.2 mol % or greater. 
     
     
       12. The fiber according to  claim 10 , wherein each core is doped with a different concentration of germania such that the absolute difference in dopant concentration between the cores is about 1.6 mol % or greater, and further wherein the cores are configured such that the distance between the center of the cores is about 55 μm and the cladding has an outer diameter of about 125 μm. 
     
     
       13. An optical fiber comprising:
 a first core, a second core, and a cladding surrounding both of the cores, wherein at least one of the following conditions is satisfied: 
 (i) each core is configured with a refractive index profile that is substantially uniform along its length and differs from the refractive index profile of the other core; 
 (ii) each core is configured with a doping concentration that differs from the doping concentration of the other core; 
 (iii) each core has a different composition than the other core; 
 (iv) the optical fiber further comprises a refractive index ring configured to surround at least one of the cores; 
 (v) the cores are configured such that the distance between the center of the cores is at least 25 μm; and 
 (vi) the cores are configured such that the distance between the center of the cores is at least 25 μm and each core is surrounded by a low refractive index ring; 
 wherein each core is doped with a different concentration of germania such that the absolute difference in dopant concentration between the cores is about 0.5 mol % or greater. 
 
     
     
       14. An optical fiber comprising:
 a first core, a second core, and a cladding surrounding both of the cores, wherein at least one of the following conditions is satisfied: 
 (i) each core is configured with a refractive index profile that is substantially uniform along its length and differs from the refractive index profile of the other core; 
 (ii) each core is configured with a doping concentration that differs from the doping concentration of the other core; 
 (iii) each core has a different composition than the other core; 
 (iv) the optical fiber further comprises a refractive index ring configured to surround at least one of the cores; 
 (v) the cores are configured such that the distance between the center of the cores is at least 25 μm; and 
 (vi) the cores are configured such that the distance between the center of the cores is at least 25 μm and each core is surrounded by a low refractive index ring; 
 wherein each core is doped with a different concentration of germania such that the absolute difference in dopant concentration between the cores is about 1.2 mol % or greater. 
 
     
     
       15. An optical fiber comprising:
 a first core, a second core, and a cladding surrounding both of the cores, wherein at least one of the following conditions is satisfied: 
 (i) each core is configured with a refractive index profile that is substantially uniform along its length and differs from the refractive index profile of the other core; 
 (ii) each core is configured with a doping concentration that differs from the doping concentration of the other core; 
 (iii) each core has a different composition than the other core; 
 (iv) the optical fiber further comprises a refractive index ring configured to surround at least one of the cores; 
 (v) the cores are configured such that the distance between the center of the cores is at least 25 μm; and 
 (vi) the cores are configured such that the distance between the center of the cores is at least 25 μm and each core is surrounded by a low refractive index ring; 
 wherein each core is doped with a different concentration of germania such that the absolute difference in dopant concentration between the cores is about 1.6 mol % or greater, and further wherein the cores are configured such that the distance between the center of the cores is about 55 μm and the cladding has an outer diameter of about 125 μm. 
 
     
     
       16. An optical fiber comprising:
 a first core, a second core, and a cladding surrounding both of the cores, wherein at least one of the following conditions is satisfied: 
 (i) each core is configured with a refractive index profile that is substantially uniform along its length and differs from the refractive index profile of the other core; 
 (ii) each core is configured with a doping concentration that differs from the doping concentration of the other core; 
 (iii) each core has a different composition than the other core; 
 (iv) the optical fiber further comprises a refractive index ring configured to surround at least one of the cores; 
 (v) the cores are configured such that the distance between the center of the cores is at least 25 μm; and 
 (vi) the cores are configured such that the distance between the center of the cores is at least 25 μm and each core is surrounded by a low refractive index ring; 
 wherein the fiber is further configured such that crosstalk between the cores is −20 dB/km or less. 
 
     
     
       17. The fiber according to  claim 1 , wherein the distance between the center of the cores is at least 40 μm. 
     
     
       18. The fiber according to  claim 16 , wherein each core is configured to exhibit a single Brillouin frequency shift peak (BFS peak) when light is launched into the core, and further wherein the BFS peak of each core differs from the BFS peak of the other core in terms of frequency. 
     
     
       19. An optical fiber comprising: a first core, a second core, and a cladding surrounding both of the cores, wherein at least one of the following conditions is satisfied:
 (i) each core is configured with a refractive index profile that is substantially uniform along its length and differs from the refractive index profile of the other core; 
 (ii) each core is configured with a doping concentration that differs from the doping concentration of the other core; 
 (iii) each core has a different composition than the other core; 
 (iv) the optical fiber further comprises a refractive index ring configured to surround at least one of the cores; 
 (v) the cores are configured such that the distance between the center of the cores is at least 25 μm; and 
 the cores are configured such that the distance between the center of the cores is at least 25 μm and each core is surrounded by a low refractive index ring, wherein each core is configured to exhibit a single Brillouin frequency shift peak (BFS peak) when light is launched into the core, and further wherein the BFS peak of each core differs from the BFS peak of the other core in terms of frequency; 
 wherein each core is configured to exhibit a Brillouin frequency shift of at least 30 Mhz relative to the other core. 
 
     
     
       20. The optical fiber according to  claim 19 , wherein the cores are single mode cores at a 1550 nm wavelength.